In the drive-train of a vehicle the drive aggregate, which in the case of a hybrid drive comprises an internal combustion engine and an electric machine, can be connected via a transmission device to a drive output. In the transmission device a hydraulically actuated clutch is often used, by virtue of which the torques of the internal combustion engine or electric machine are transferred to the drive output when the clutch is closed. To actuate the clutch and appropriate shifting elements in the transmission device a hydraulic fluid circuit is provided, which by means of a hydraulic pump provides a system pressure sufficient for actuating the clutch and for engaging gears.
The document DE 10 2008 040 667 A1 describes a hydraulic system of a transmission device for a vehicle, with a main transmission pump and an auxiliary pump. The main transmission pump is powered primarily by the internal combustion engine and the auxiliary pump by an electric machine of the vehicle.
The document DE 10 2008 044 272 A1 describes a method for operating a hybrid drive-train of a vehicle which comprises a transmission device with hydraulically actuated shifting elements. The transmission device has a hydraulic system with an oil pump device which can be driven both by the internal combustion engine and by the electric machine of the drive-train.
If a hydraulic fluid circuit with a hydraulic pump is used in a transmission device of a drive-train, in which the pump can be operated by both an electric machine and also by the internal combustion engine of the drive-train, the filling of the clutch by means of the hydraulic pump while the internal combustion engine is at first switched off takes place by means of the electric machine. During this the electric machine operates without taking account of a driver's wish.